This invention relates to cresolphthalein monophosphoric acid and salts thereof, a process for their production, and use of the salts as substrates for alkaline phosphatase determinations.
As depicted in Formula I and III, the cresol moiety is ortho-cresol. Accordingly, the term "cresolphthalein" appearing throughout the specification is to be construed as meaning ortho- or o-cresolphthalein.
Phenolphthalein monophosphates, as disclosed in U.S. Pat. Nos. 3,331,857 and 3,331,862, and thymolphthalein monophosphates hitherto have been employed advantageously as substrates for determining phosphatase. The amount of phenolphthalein or thymolphthalein released by enzymatic hydrolysis of the corresponding ester is proportional to the enzyme concentration. While both of the materials constitute popular substrates, they have certain limitations. Thus, phenolphthalein-based tests show some interference from bilirubin. Thymolphthalein monophosphate shows some lack of sensitivity to intestinal and placental phosphatase isoenzymes. Thymolphthalein monophosphate also is not suited for alkaline phosphatase determination by the kinetic method, described hereinafter.
It would be desirable to provide substrates for alkaline phosphatase determinations having the advantages of the prior phthalein monophosphates while minimizing or obviating the disadvantages thereof.